First you've got to work out why you dropped, then you need to address it.

Questions>
What have you done to your site recently or immediately prior to the drop?
-make a list, share it here

Have all pages that previously 'ranked' page one dropped, if not which ones?
- describe the sort of pages which dropped, are they content pages, directory, products, news?

Are you running the sort of website that may have fallen foul of Google's Panda update?
- this means spammy, duplicated content, low user value website or a website relying on such websites for links. Also some websites that relied on long tail keywords for organic traffic or websites that have a specific keyword in the domain name.